Interpretive Summary: Cheatgrass is an exotic annual grass causing ecosystem degradation in western United States rangelands. We investigated potential mechanisms by which the perennial, crested wheatgrass, suppresses cheatgrass. Research focused on monthly extractable (mineral) soil nitrogen (N) and the proportional concentration of ammonium-N in a crested wheatgrass community by microsite (crested wheatgrass, un-vegetated interspace, shrub subcanopy) and soil depth (0-15, 15-30 cm) over a one-year period. Extractable soil N in crested wheatgrass microsites ranged from 0.24 to 1.66 mmol kg-1 and was not appreciably lower than other ecosystems we have measured in the Great Basin. The molar proportion of ammonium-N in the extractable soil pool beneath crested wheatgrass averaged over 85% for the year and is far greater than other plant communities we have measured in the Great Basin. We conclude that crested wheatgrass does not suppress cheatgrass by controlling extractable N below a threshold level; rather, we hypothesize that it may limit the conversion of ammonium-N to nitrate-N and thereby reduce nitrate-N availability to cheatgrass.

Technical Abstract:
Cheatgrass (Bromus tectorum L.) is an exotic annual grass causing ecosystem degradation in western United States rangelands. We investigated potential mechanisms by which crested wheatgrass (Agropyron cristatum [L. Gaertn.], A. desertorum [Fisch. (Ex Link) Scult.]) suppresses the growth and invasibility of cheatgrass. Research focused on monthly extractable (mineral) soil nitrogen (N), net N mineralization potentials, and the proportional concentration of ammonium-N in a crested wheatgrass community by microsite (crested wheatgrass, un-vegetated interspace, shrub subcanopy) and soil depth (0-15, 15-30 cm) over a one-year period. Extractable soil N in crested wheatgrass microsites ranged from 0.24 to 1.66 mmol kg-1 and was not appreciably lower than the other microsites or other ecosystems we have measured in the Great Basin. The molar proportion of ammonium-N in the extractable pool of crested wheatgrass averaged over 85% for the year and is significantly higher than the other microsites and far greater than other plant communities we have measured in the Great Basin. We conclude that crested wheatgrass does not suppress cheatgrass by controlling extractable N below a threshold level; rather, we hypothesize that it may limit nitrification and thereby reduce nitrate-N availability to the nitrophile cheatgrass.